1. Field of the Invention
The present invention relates to a light-emitting device having a laminate of an organic EL layer and a photoconductive layer, and relates to an exposure device and a display device using the light-emitting device.
2. Description of the Related Art
Heretofore, Braun tubes (cathode-ray tubes) have been used in a large-size information display device exclusively. When information era comes and particularly, portability is regarded as important, flat-plate-like display devices which are thin in thickness and light in weight and which is low in operating voltage are in demand. A liquid-crystal display device is one of such flat-plate-like display devices. However, because of a back-lighting system required, the liquid-crystal display device is increased correspondingly in consumed electric power and has a limitation in reduction of thickness.
An organic EL (electroluminescent) device is a flat display device which can operate with a low voltage and which can be made thin because of no back-lighting system required. For example, a display device having a simple matrix structure shown in FIG. 8 has been put to practical use as a display device including an organic EL device 1. In the simple matrix structure, a number, X, of pixels connected to row electrodes 3 and arranged laterally in one row are selected so as to operate simultaneously. An on-off state of each pixel is set in accordance with a display information signal by switching devices connected to a number, Y, of column electrodes 5 respectively. When adjacent ones of the row electrodes 3 are selected sequentially so as to go through the whole display screen once, one scene is displayed.
For example, in an NTSC system, however, the scene is rewritten and displayed at the rate of 30 scenes per second or more (N scene/sec) in order to obtain a flickerless screen with an animation image displayed by the aforementioned scanning method. Hence, the period of time that the selected pixels are in an operating state is only 1/(Nxc2x7Y) sec. For example, in the case of N=30 and Y=300, the period of time is {fraction (1/9000)} sec. In this case, voltage application for a plurality of cycles cannot be performed in one selection period if the frequency of a driving electric source is not higher than tens of kHz. A pulse voltage driving method in which a pulse voltage is given in synchronism with the selection period is, however, used generally as a practical method for driving respective pixels on one and the same condition. Hence, this is equivalent to the fact that the display device is driven by the frequency N, so that no higher luminance but luminance corresponding to the frequency can be expected.
In order to satisfy the necessary luminance (=average luminancexc3x97the number of scanning lines), a large peak current flows in the row electrodes 3, the column electrodes 5 and the organic EL devices 1. As a result, a voltage drop caused by the large peak current makes the image uneven. Moreover, because the luminance required of each organic EL device 1 increases, a burden for retrieving materials is needed and the life and stability is reduced. These disadvantages become more remarkable as the definition and screen of the display device become higher and larger respectively.
To eliminate the aforementioned disadvantages, therefore, a display panel having TFT such as a-Si or p-Si provided for each pixel has been described in JP-A-8-234683. In the display panel shown in FIG. 9, when a scanning signal is supplied to a selected row electrode 3, a first TFT 7 is turned on so that a capacitor 9 is charged with the image signal voltage applied to the column electrode 5. This signal voltage becomes a gate-source voltage of a second TFT 11. Hence, a constant current corresponding to the gate-source voltage flows in the organic EL device 1 connected to the drain of the second TFT 11, and goes from a first common electrode 13 toward a second common electrode 15. As a result, the organic EL device 1 emits light with luminance approximately proportional to the current value.
Even in the case where the first TFT 7 is now turned off in order to select another row, the luminance of light emission is kept constant until all scans are terminated and the selection scanning of the row can be performed again because the image signal voltage is stored in the capacitors 9 (memory characteristic attained by the first TFT and the capacitors). Such memory characteristic attained by the TFT can be provided to thereby solve the problem of low luminance or the like in simple matrix driving.
As another display device provided to eliminate the aforementioned disadvantages, a display device attaining the memory characteristic without use of any TFT has been described in JP-A-9-185332. The display device shown in FIG. 10 generally comprises an EL display device 1 for display light emission, and an address light device 17 for scanning and signal writing, disposed on the back of the EL display device 1. The EL display device 1 includes, when viewed from the back, an electrode 19, a photoconductive layer 21, a light-emitting layer 23, and an electrode 25 in sequence. The address light device 17 for scanning and signal writing is formed as a simple matrix structure.
When a voltage is applied between row electrodes 3 selected by linearly sequential scanning and column electrodes 5 in the address light device 17, the light-emitting layer 27 is irradiated with ultraviolet light as signal light toward the photoconductive layer 21. A portion of the photoconductive layer 21, on which the signal light is incident, forms an electric conductor by absorbing the light. Hence, the portion of the photoconductor layer 21 is electrically connected to the rear driving electrode 19. Hence, the electric potential applied between the front driving electrode 25 and the rear driving electrode 19 is applied to a predetermined dot portion of the light-emitting layer 23.
As a result, forward light is emitted and, at the same time, backward light, that is, feedback light is emitted because of light emission of the EL display device 1. The feedback light enters the photoconductive layer 21, so that the photoconductive layer 21 is excited again to thereby generate new electron-hole pairs. Hence, the photoconductive layer 21 is kept in an electric conduction state, so that the driving voltage is continuously applied to a corresponding portion of the EL display device 1. Because the display light-emitting layer is always driven during this state, the display light-emitting layer is kept in a light emission state even in the case where the column and row electrodes 5 and 3 in the address light device 17 are not in a selected state. Hence, light emission is continued by linearly sequential scanning until the second scanning cycle.
Hence, memory characteristic can be achieved by feedback light in each pixel in a simple structure without use of active driving represented by TFT, so that high-quality display can be performed under a high-duty-cycle driving condition.
As a further display device provided to eliminate the aforementioned disadvantages, a display device attaining memory characteristic by optical switches of bipolar transistors has been disclosed in JP-A-10-171375. The display device shown in FIG. 11 comprises a glass substrate 31, an npn-structure bipolar transistor portion 33 formed on the glass substrate 31, and an organic EL device 1 formed on the bipolar transistor portion 33. A rear electrode 35 is formed on the back surface of the bipolar transistor portion 33. A rear electrode 35 and a front electrode 37 forming an XY matrix are formed on the front surface of the organic EL device 1. Hence, when a predetermined voltage is applied between the front and rear electrodes, the organic EL device 1 emits light to thereby generate display light and, at the same time, the bipolar transistor portion 33 is excited optically so that memory characteristic can be given to the light emission.
Hence, also in this display device, memory characteristic can be achieved by feedback light in each pixel in a simple structure without use of any TFT or the like. Hence, high-quality display can be performed under a high-duty-cycle driving condition.
The display panel having TFT provided for each pixel as shown in FIG. 9, however, has a problem that the production cost is high because the number of production steps increases for the formation of TFTs. In addition, it is difficult to form TFTs adapted to a large-screen display device for TV having a size exceeding 30 inches.
In the display device having memory characteristic attained by positive feedback of light without use of any TFT as shown in FIG. 10, if a general photoconductor and an organic EL layer are laminated on each other as a structure of the address light device for scanning and signal writing and the EL display device for display light emission, it is necessary to generate at least one electron or hole from the photoconductive layer when the quantum efficiency of the display light emission EL layer is not larger than 1, so that the display light emission EL device can emit one photon. In other words, when the photoelectronic quantum efficiency of the photoconductive layer is not larger than 1, the number of light emission photons required for the address light device to perform scanning and signal writing is not smaller than 1. Generally, the aforementioned quantum efficiency is not larger than 1, and if the wavelength of light absorbed to the photoconductive layer is shorter than that of display light, display light emission suffers an energy loss out of the light absorbed to the photoconductive layer.
Hence, it is conceived that it is difficult to keep the display light emission state the same as that at the write time even in the case where light emission of the address light device for scanning and signal writing is stopped and light emitted from the EL display device for display light emission is absorbed to the photoconductive layer. Moreover, also at the scanning/writing time of the address light device for scanning and signal writing, the address light device for scanning and signal writing is required to emit light with sufficient energy for the aforementioned reason. Hence, the address light device for scanning and signal writing in a simple matrix structure has still the aforementioned problem (peak current). In addition, memory characteristic attained by positive feedback of light is low in stability and there is a problem that it is difficult to obtain halftone characteristic.
The display device having memory characteristic attained by optical switches of bipolar transistors as shown in FIG. 11 is a device in which light emitted backward from the organic EL layer (feedback light) is absorbed to a base of a bipolar transistor to thereby perform current conduction of the bipolar transistor on the basis of the photocurrent thereof to thereby feedback the current flowing in the organic EL layer to thereby give memory characteristic. Similar to the case of TFTs, the number of production steps for forming bipolar transistors increases and the production cost is high. There is a problem that it is difficult to form a large-screen display device. In addition, it is difficult to put the organic EL device into practical use because n-type semiconductor is unstable in air.
The present invention is designed upon such circumstances and an object thereof is to provide a light-emitting device in which: the burden imposed on the light-emitting device can be lightened by use of a simple matrix structure; stable light emission with memory characteristic can be made as in the case where TFT is used; and high definition and large display screen can be achieved inexpensively; and to provide an exposure device and a display device using the light-emitting device.
In order to achieve the above object, according to an aspect of the present invention, there is provided a light-emitting device comprising: a display light-emitting part made of a laminate of a first electrode, an organic EL layer, a photoconductive layer and a second electrode in sequence; and an external light-emitting part laminated on the second electrode side of the display light-emitting part, the photoconductive layer of the display light-emitting part receiving light emitted from the external light-emitting part, the photoconductive layer being made of a photoconductor having a current amplification mechanism so that carriers generated in a photoelectric conversion process are multiplied by the current amplification mechanism.
In the light-emitting device, when the external light-emitting part emits light, the light enters the photoconductive layer of the display light-emitting part. Hence, carriers are generated in the light-incidence region of the photoconductive layer, so that light emits from a region of the display light-emitting part corresponding to the region of the photoconductive layer in which the carriers are generated. On this occasion, the carriers generated in a photoelectric conversion process are multiplied by the current amplification mechanism. Hence, the carrier generating efficiency exceeds 1, so that high luminance is obtained.
According to another aspect of the present invention there is provided a light-emitting device comprising: a display light-emitting part made of a laminate of a first electrode, an organic EL layer, a photoconductive layer and a second electrode in sequence; and an external light-emitting part laminated on the second electrode side of the display light-emitting part, the photoconductive layer of the display light-emitting part receiving light emitted from the external light-emitting part, the photoconductive layer being made of a photoconductor having a current amplification mechanism so that carriers generated in a photoelectric conversion process are multiplied by the current amplification mechanism and continuously generated by light emitted from the organic EL layer.
In the light-emitting device, light is emitted from the display light-emitting part to the outside and to the photoconductive layer. The feedback emitted light is absorbed to the photoconductive layer again, so that a great deal of carriers are generated by the aforementioned multiplication function. Hence, even in the case where incident light from the external light-emitting part is turned off, carriers are continuously generated due to the feedback light effect. There is therefore provided memory characteristic that light is continuously emitted from the display light-emitting part until the voltage is turned off.
In the light-emitting device in this invention, the photoconductor has a current amplification mechanism attained by an avalanche effect.
In the light-emitting device, when a high intensity of electric field is applied to the photoconductor, a carrier collides with a grating atom to ionize bound electrons to thereby generate free electrons and holes as the secondary carriers. These secondary carriers take part in collision ionization sequentially, so that multiplication of carriers occurs acceleratively. Hence, rapid multiplication of current occurs in the photoconductor.
In the light-emitting device in this invention, the photoconductor is one member selected from the group consisting of c-Si, a-Se and a-SiC.
In the light-emitting device, any one of c-Si, a-Se and a-SiC is used as the material of the photoconductor so that an avalanche effect can be generated in the photoconductive layer.
In the light-emitting device in this invention, the photoconductor has a current amplification mechanism by which carriers (electrons or holes) are injected from the second electrode into the photoconductive layer on the basis of carriers (holes or electrons) generated in the photoconductive layer by light absorption.
In the light-emitting device, when holes are generated in the photoconductive layer by light absorption, electrons are injected from the second electrode into the photoconductive layer by the current amplification mechanism of the photoconductor. Further, when electrons are generated in the photoconductive layer by light absorption, holes are injected from the second electrode into the photoconductive layer.
In the light-emitting device in this invention, injection of the carriers exhibits a tunnel current in an interface between the photoconductive layer and the second electrode.
In the light-emitting device, carriers not having sufficient energy to skip over the potential barrier can pass through the barrier on the basis of the wave function of carriers taking a large value on the opposite side to the barrier because the potential barrier is thin. Hence, a current flows in the interface between the photoconductive layer and the second electrode.
In the light-emitting device in this invention, each of the carriers to be injected is an electron.
In the light-emitting device, holes are generated in the photoconductive layer by light absorption, so that electrons are injected from the second electrode into the photoconductive layer.
In the light-emitting device in this invention, the photoconductive layer is made of an organic photoconductor.
In the light-emitting device, the current multiplication phenomenon is obtained by the photoconductive layer made of an organic thin film even without the avalanche effect obtained by inorganic silicon semiconductor or the like.
In the light-emitting device in this invention, the organic photoconductor is one member selected from the group consisting of a perylene pigment, a quinacridone pigment, and a naphthalenetetracarboxylic derivative.
In the light-emitting device, a perylene pigment, a quinacridone pigment, a naphthalenetetracarboxylic derivative, or the like, can be used to form the photoconductive layer of an organic thin film so that the current multiplication phenomenon can be obtained.
In the light-emitting device in this invention, the second electrode is an Au electrode.
In the light-emitting device, an Au electrode is used. Particularly when the Au electrode serves as a cathode, carriers (electrons) can be injected easily from the Au electrode to thereby improve the current multiplication effect.
In the light-emitting device in this invention, injection of the carriers is based on skipping over an electric potential barrier.
In the light-emitting device, carriers generated by light irradiation are trapped in the interface between the second electrode and the photoconductive layer. Hence, the barrier to injection of carriers is lowered to promote injection of carriers from the second electrode and increase the photocurrent.
In the light-emitting device in this invention, each of the carriers to be injected is a hole.
In the light-emitting device, carriers are generated in the irradiation portion of the photoconductive layer. Similarly, holes move to the first electrode side and electrons are trapped in the interface between the photoconductive layer and the second electrode. Hence, in the irradiation portion of the photoconductive layer, the electron density of the interface between the photoconductive layer and the second electrode increases in comparison with that in the non-irradiation portion. Hence, in the irradiation portion, injection of holes from the second electrode side increases, so that the photocurrent is amplified by the injection of holes.
In the light-emitting device in this invention, the photoconductive layer is made of an organic photoconductor.
In the light-emitting device, the current multiplication phenomenon is obtained by the photoconductive layer made of an organic thin film even without the avalanche effect obtained by inorganic silicon semiconductor or the like.
In the light-emitting device in this invention, the organic photoconductor is a bis-azo pigment.
In the light-emitting device, a bis-azo pigment can be used to form the photoconductive layer so that the current multiplication phenomenon owing to skipping over the potential barrier can be obtained.
In the light-emitting device in this invention, the second electrode is made of an ITO film.
In the light-emitting device, the second electrode is formed from an ITO film. The ITO film is an electric conductor in which transparency can be controlled freely in a range of from near-ultraviolet light to visible light and near-infrared light. Particularly when the electrode serves as an anode, carriers (holes) can be injected easily from the electrode to thereby improve the current multiplication effect.
In the light-emitting device in this invention, light emitted from the organic EL layer is contained in a sensitive wavelength range of the photoconductor.
In the light-emitting device, light emitted from the organic EL layer is contained in the sensitive wavelength range of the photoconductor. Hence, carriers are generated in the photoconductor by feedback light from the organic EL layer.
In the light-emitting in this invention, the external light-emitting part is made of organic EL layer having a simple matrix structure.
In the light-emitting device, when the external light-emitting part emits light in any matrix intersection position of the simple matrix structure, a region of the photoconductor corresponding to the light-emitting position is irradiated with light to thereby generate carriers. Hence, the same region of the display light-emitting part can be displayed by simple matrix driving.
In the light-emitting in this invention, row selection scanning light is emitted from the external light-emitting part; and an image signal is supplied to column electrodes of the organic EL layer.
In the light-emitting device, when row selection scanning light is emitted from the external light-emitting part, carriers are generated in the row direction of the photoconductor. On the other hand, an image signal is supplied to a corresponding column electrode of the organic EL layer. Hence, the organic EL layer emits light at a point of intersection between the row irradiated with the row selection scanning light and the column to which the image signal is applied.
In the light-emitting in this invention, a row selection scanning signal is supplied to row electrodes of the display light-emitting part; and image signal light for column selection is emitted from the external light-emitting part.
In the light-emitting device, when column selection image signal light is emitted from the external light-emitting part, carriers are generated in the column direction of the photoconductor in accordance with the image signal. On the other hand, a row selection scanning signal is supplied to a corresponding row electrode of the organic EL layer. Hence, the organic EL layer emits light at a point of intersection between the column to which the column selection image signal is applied and the row irradiated with the row selection scanning signal light.
In the light-emitting in this invention, the organic EL layer is made of a high-molecular material.
In the light-emitting device, a high-molecular compound made from a polymer can be used as the organic EL layer.
In the light-emitting in this invention, the organic EL layer is made of a low-molecular material.
In the light-emitting device, a low-molecular compound can be used as the organic EL layer.
In the light-emitting device in this invention, the organic EL layer is of a single layer type.
In the light-emitting device, a homogenous light emission color can be obtained in an easy production process.
In the light-emitting device in this invention, the organic EL layer is of a laminate layer type.
In the light-emitting device, light-emitting layers with different colors can be laminated on each other. When the light-emitting layers are driven independently, light with different mixture colors can be emitted in a predetermined region.
In the light-emitting in this invention, a resistance layer is provided between voltage-applied electrodes.
In the light-emitting device, a high intensity of electric field necessary for EL light emission can be applied to the organic EL layer stably without causing any insulation breakdown between the first and second electrodes. The electric field can be controlled so that a constant current flows in the organic EL layer. Moreover, the organic EL layer can be insulated from the atmosphere to thereby improve stability.
According to a further aspect of the present invention, there is provided an exposure device comprising a light-emitting device, the light-emitting device being used for exposing a photosensitive material to light.
In the exposure device, the light-emitting device is driven on the basis of image information. Hence, a photosensitive material can be exposed to light emitted from the light-emitting device. Moreover, digital multiple exposure can be made, so that high-speed recording can be made.
According to another aspect of the present invention, there is provided a flat display device comprising a light-emitting device, the light-emitting device being used for displaying any optional image.
In the flat display device, the light-emitting device is driven on the basis of image information. Hence, light emission of the organic EL layer can be controlled by writing with low energy. Light emission of the display light-emitting part can be continued stably by carriers multiplied to be not smaller than the light carriers generated by light emission of the display light-emitting part per se. Two-dimensional light emission display attained by memory characteristic can be made in the same level as the case where TFT is used.